Noob Answer: Try checking your audio settings in game and see if all of the bars are high.
OR
In your GTA:SA directoy, which should look something like \Rockstar Games\GTA San Andreas\, in the root directory, you'll find a file called "eax.dll". It would appear that this file is the problem.
Now, go onto google.com and search for eax.dll. You should find one which is version 3.0.6.6. (Funnily enough, the version that GTA:SA comes with is version 4.0.0.1)
Once you've downloaded "eax.dll" 3.0.6.6, rename the "eax.dll" in your GTA:SA directory to something else of your choosing, then copy the "eax.dll" that you just downloaded into the GTA:SA directory. At this point, if you've turned down your hardware acceleration, turn it back on.
Load up GTA:SA, and you should find that your audio problems are now GONE!!! Woohoo!!! The audio should also sync back up with the video also. (I don't know why, but it does)
OR
Check your volume settings on the right task bar and check if they are not muted